1. Field of the Invention
The present invention relates to a surface acoustic wave element including a laminated substrate, a method for producing the same and a surface acoustic wave device using the same.
2. Description of the Related Art
In the development of mobile communication equipment, it is desired to achieve high performance of a surface acoustic wave element, which is one of the key devices constituting the equipment. In the case where a band in which signals are sent out is close to a band in which signals are received, as seen in the recent mobile communication systems, it is difficult to achieve a sharp cut-off property in the temperature range used. This is due to the characteristics of a piezoelectric substrate used in conventional surface acoustic wave elements. Specifically, this is because although conventional piezoelectric substrates have a coupling coefficient sufficient to achieve a required band width of the system, in general they have a large frequency temperature coefficient. In order to cope with this problem, it was reported that attaching an existing piezoelectric substrate to an auxiliary substrate having a thermal expansion coefficient different from that of the piezoelectric substrate provides a surface acoustic wave element having a large coupling coefficient and excellent temperature stability (Proc. 1997 IEEE Ultrasonics Symposium, pp. 227–230).
Hereinafter, a conventional surface acoustic wave element will be described. FIG. 13A is a perspective view of an example of a conventional surface acoustic wave element, and FIG. 13B is a cross-sectional view thereof taken along line Z—Z in FIG. 13A. Referring to FIGS. 13A and 13B, the conventional surface acoustic wave element includes a first substrate 401, a second substrate 402, a comb-shaped electrode 403 including electrodes 403a and 403b, and a reflector 404. As the first substrate 401, a 36° Y-cut X-propagating lithium tantalate single crystal is used, for example. As the second substrate 402, a glass substrate having a thermal expansion coefficient smaller than that of the first substrate 401 in the propagation direction of a surface acoustic wave is used, for example. The thickness of the first substrate 401 is sufficiently smaller than that of the second substrate 402, and is sufficiently larger than the wavelength of the surface acoustic wave. For example, the first substrate 401 is about 40 μm thick and the second substrate 402 is about 310 μm thick. The first substrate 401 and the second substrate 402 are joined together substantially directly without an adhesive or the like therebetween. Such a structure allows control of the frequency temperature coefficient while maintaining the characteristics of the existing piezoelectric substrate.
However, the conventional surface acoustic wave element as described above has the following problems. Since it has a laminate structure where the piezoelectric single crystal several tens of μm thick and the glass substrate are laminated, it is difficult to handle the surface acoustic wave element. For example, in mounting the surface acoustic wave element on a package, in particular, in picking up the surface acoustic wave element, cracking or fracturing may occur in the piezoelectric single crystal layer. Moreover, in dividing a wafer into individual surface acoustic wave elements, when it is cut with a cutting blade selected based on the glass substrate, fracturing or chipping may occur in the piezoelectric single crystal portion during cutting because of the difference in the characteristics between the materials.